An emerging trend in the telecommunications industry is to provide data services deployed over existing telephone twisted pair copper wires utilizing a frequency spectrum above the voice frequency band. One such type of transceiver enabled to provide data services deployed over existing telephone wires is one employing Discrete Multi-Tone (DMT) techniques. The DMT encoding makes use of a wide bandwidth channel divided up into sub-channels and data is modulated onto the sub-channels using a modulation method called quadrature amplitude modulation. The frequency band of the DMT channel is dictated by physical properties of the twisted pair wires used in providing existing telephone services and the existing infrastructure of the Public Switched Telephone Network (PSTN). The DMT frequency band of downstream transmissions extends into the frequency range of amplitude modulated (AM) radio transmissions and is, therefore, susceptible to Radio Frequency Interference (RFI) from AM radio stations.
The telephone wiring acts as a receiving antenna and converts electromagnetic energy from AM radio transmissions into a common mode voltage in the wiring. When the telephone wiring and the receiver front-end of a data transceiver are perfectly balanced, this contaminating signal should not cause any problems because of a differential mode of signal detection employed by the receiver. However, when there are front-end or wiring imbalances, a common mode voltage is detected at the receiver. As a result, AM radio signals from nearby stations within the frequency range are added to the input DMT signal and contaminate it. AM radio stations can occupy frequencies ranging from 535 kHz up to 1605 kHz with 10 kHz channelization. The effect the high frequency stations have on the received signal at the DMT transceiver can be suppressed by employing analog filters in the front-end of transceivers for partial rate services, e.g. G.lite ADSC, which is well known in the art. In this way, radio frequency interference (RFI) from radio stations above about 650 kHz is suppressed leaving the AM carriers below 650 kHz as most damaging to the received DMT signal. Analog filters cannot be used to suppress the RFI from full-rate ADSC because the full-rate implementation utilizes about twice the bandwidth of the G.lite implementation.
In the G.lite implementation, a logical solution to the damaging effects of the remaining low frequency AM RFI signals is to employ sharper analog filters or filters with lower frequency ranges to provide a better suppression of the RFI. However, there is considerable cost associated with this solution, including an increase in complexity of the transceiver and a loss of useful sub-channels in the DMT bandwidth.
Digital filters are used to complement the filtering done by the analog filters and, these are employed in order to reduce the net effect of the RFI from low frequency AM radio transmissions after the contaminated DMT signal is digitized. In order for digital filtering to be effective, the received analog signal should be optimally digitized.
Another effect that AM RFI has on the front-end processing of the received DMT signal, is that the signal characteristics are no longer predetermined. This is due to the influence of geographical location on the mix of RFI frequencies and their relative power levels. The unpredictable nature of the characteristics of the received RFI contaminated DMT signal leads to an inability of prior art transceivers to optimally digitize the received signal resulting in a higher quantization error than necessary being introduced in the digitization process.
A logical solution to the inability to optimally digitize the received DMT signal is to employ analog-to-digital converters of a higher resolution. However, an analog-to-digital converter of a higher resolution adds cost and complexity to the DMT transceiver.
In order to deploy data services on existing telephone twisted pair wires, there exits a need for the development of new apparatus and methods for AM RFI suppression using digital filtering anid, in particular, for an apparatus and method of reducing quantization noise in the digitization process.